The present invention relates to gas analysers, such as an infrared (ray) gas analyser of the non-dispersion type or an ultraviolet (ray) gas analyser, which are used for measuring the concentration of carbon monoxide or the like in the air.
Convenional infrared gas analysers of the non-dispersion type are shown in FIG. 1 and FIG. 2. FIG. 1 shows a gas analyser of the non-dispersion type which adopts a method employing a double light path and intermittant light and which consists of light source 44, 44, revolving sector 45, reference cell 46, measurement cell 47 and detector 41. Although various kinds of detector 41 have been used, the present discussion will make reference to a pneumatic detector using a condenser microphone. In order to eliminate the influence of a change in the surrounding temperature upon detector 41, the separated right and left rooms 42a and 42b, respectively, are communicated by an aperture or leak 43 so that the pressure therebetween will always be at a static equilibrium and thus detector 41 will sense only dynamic pressure changes having short cycles. In order to realize this, there is installed a revolving sector 45 which intermittently passes an infrared ray emitted from light source 44, 44 at constant intervals. Furthermore, gases such as nitrogen which do not absorb infrared rays are enclosed in reference cell 46. Zero gas is put into sampling cell 47, and then the energy of the infrared rays reaching the right and left rooms 42a and 42b of detector 41 are balanced and their phases are so equalized that the output of detector 41 is adjusted to zero. Then, the test gas is put into sampling cell 47. If the test gas absorbs the energy of infrared rays while the rays pass through sampling cell 47, a difference occurs between the energy of the infrared rays which pass through sampling cell 47 and of the infrared rays which pass through reference cell 46. This difference of energies leads to the generation of an unbalanced pressure signal synchronized with the cycles of revolving sector 55 between room 42b and room 42a of detector 41. Such signal is amplified and indicated on indicator 48 as a measurement of the concentration of the specified gaseous component in the test gas.
However, in this method a slight break or change in the energy balance between right room 42a and left room 42b of the optical system eliminates stability in a highly sensitive region. The reasons why this method is not suitable for measuring a particularly small amount of a gaseous component are because a remarkably high precision is required to maintain a balance of energy, an equalization of phases or the like (zero adjustment of detector 41), the adjustment is troublesome and time-consuming and furthermore an expensive apparatus is required for carrying out the adjustment. In addition to the above, an additional problem exists with regard to maintenance because the device has mechanically movable parts.
FIG. 2 shows a single light path gas analyser of the non-dispersion type in which the light intermittent method is not adopted without using a reference cell and a revolving chopper described in the above mentioned example. In this arrangement, 51 is a sampling cell and 58 is a detector. Although various kinds of detector 48 have been used, the present discussion will make reference to a pneumatic detector. The test gas and the standard gas (for example, zero gas) are alternately put into sampling cell 51 by operating pressure regulators 53a, 53b and needle valves 54a, 54b, respectively, in addition to by alternately opening and closing three-way electro-magnetic valves 52a, 52b. Infrared rays emitted from a light source 55 are not absorbed while the sampling cell is filled with zero gas. On the other hand, the special gaseous component in test gas absorbs infrared rays when the test gas is put into sampling cell 51, and thus a condenser membrane 57 provided in the separated room 56 is pressurized. The static capacity of the condenser is altered at a constant cycle synchronized with a change-over cycle of three-way electro-magnetic valves 52a, 52b. The concentration of the gaseous component is measured by electrically measuring such change in the static capacity of the condenser.
But, although this single-cell type device can overcome to some degree the defects of the above mentioned prior art gas analyser of FIG. 1, the length of the sampling cell must be increased in order to measure special gaseous components which are contained in particularly small amounts in the test gas because the quantity of infrared rays absorbed is in proportion to the length of the cell and thus also the space for receiving gas in the sampling cell 51 is increased. Therefore, the quantity of the test gas or zero gas put into sampling cell 51 is increased enormously for the measurement of small amounts of gaseous components. For example, in the measurement of carbon monoxide in air, a length L of the cell of 30 to 50 cm and a space V for receiving gas of 90 to 150 cm.sup.3 are normally required. If a frequency of 5 Hz is used in detector 58, the test gas or zero gas must be put into sampling cell 51 at a ratio of 27 to 45 liters/min. Thus, a pump of a great capacity is required and therefore a large scale apparatus is required. This leads to the problem of large costs. A gas analyser of this type has the defect in that it has no practical use because a supply of zero gas of a great volume is required in addition to the above mentioned troublesome problems.